Fix bug where E438 won't evaluate correctly

[Teester]

Necessity was calculated incorrectly when a shape had only 1 expression as there was no expressions key in the shape, just a single expression key. This takes that into account.

Also added a test for this issue

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.17%. Comparing base (0d13754) to head (4b74794).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   90.08%   90.17%   +0.09%     
==========================================
  Files           6        6              
  Lines         958      967       +9     
==========================================
+ Hits          863      872       +9     
  Misses         95       95              

Files	Coverage Δ
comparejsonld.py	96.53% <100.00%> (+0.07%)	⬆️
test_schemas.py	98.47% <100.00%> (+0.03%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0d13754...4b74794. Read the comment docs.